A conventional drilling tool or hole saw includes a cylindrical cutter to form a relatively large round hole in a workpiece such as an iron plate. The hole saw has a shank mounted to and driven by a hand-held drive unit such as an electric motor.
An operator holds the hand-held drive unit to press the cylindrical cutter against a workpiece during a drilling operation. However, it is not so easy to form a desired round hole accurately in a predetermined position. This is because a reaction force is created when the cylindrical cutter comes into contact with the workpiece while the cylindrical cutter is being rotated. This results in displacement of the cutter from the workpiece. Also, if the cylindrical cutter is not advanced in a direction perpendicular to the workpiece, then the resultant hole is ellipsoidal in shape, rather than round.
It is also well known to mount a drill centrally to a cylindrical cutter so that its proximal end extends beyond the cylindrical cutter. In the prior art, the drill is first inserted into a workpiece and serves as a guide. The cylindrical cutter is then moved toward the workpiece. This provides for accurate positioning of the tool and formation of a desired round hole.
If an iron plate as a workpiece is relatively thin, the conventional tool is able to form a desired hole while it is guided by the drill. However, the drill may adversely affect drilling operation if an iron plate as a workpiece is relatively thick.
When the both drill and the cylindrical cutter are operated to form holes in a thick workpiece, substantial resistance is applied to the drilling tool. This causes a hand-held drive unit to suffer from torque shortage. A substantial amount of time is thus required to complete a drilling operation.
An operator must press the drilling tool strongly against the workpiece to apply sufficient thrust to overcome such substantial resistance. This results in hard work and thus, fatigue of the operator. Also, the drill may readily be damaged or broken if the drilling tool is displaced while it is advanced into the thick workpiece.
Additionally, in the event that a selected workpiece has a curved surface, such as a pipe, it is difficult to immovably hold a hand-held drive unit during a drilling operation so as to prevent displacement of the drill in the direction in which the drilling tool is advanced. The drilling tool is inevitably swung and displaced to often cause breakage of the drill as mentioned above.